Friction-Fusion Welding Of Strap PA0 Torsion Bar Friction-Fusion Strap Welding Machines PA0 Conventional Torsion Bar Machine Design PA0 Problems With Low Amplitude Torsion Bar Oscillation PA0 Problems With High Amplitude Strap Oscillation PA0 Problems With Curved Welding Areas PA0 Chain Assemblies for Slip-Feed Strapping Machines PA0 Friction-Fusion Welding Of Thin Film Wide Strap PA0 Friction-Fusion Weld Pad Design PA0 Torsion Actuation System PA0 Lug and Chain Assembly PA0 Anvil, Gripper, and Cutter Assembly
A variety of methods have been marketed and/or proposed over the years for securing together overlapping portions of a tensioned loop of thermoplastic strap encircling an article. One method is effected by first compressing the overlapping strap portions together and then creating a unidirectional or multidirectional bodily sliding frictional movement between the contacting surface regions of the overlapping strap portions to melt interface regions of the overlapping strap portions. The melted interface regions are allowed to solidify so as to bond the overlapping strap portions together.
This process, which can be generally described by the term friction-fusion welding, has proven to be especially effective with conventional thermoplastic strap materials such as nylon, polyester, polypropylene, and the like. Such conventional strap is typically provided in widths ranging from 5 mm. to 9.525 mm. and has a thickness ranging between about 0.254 mm and about 0.889 mm.
In conventional tools and machines for effecting a friction-fusion weld between overlapping portions of thermoplastic strap, an engaging member is provided for gripping the outwardly directed surface of one of the overlapping strap portions and an anvil is provided for contacting the outwardly directed surface of the other overlapping strap portion. The strap engaging surface of the engaging member and of the anvil may each be planar or may each be curved for receiving the overlapping strap portions. Relative movement is effected between the engaging member and the anvil so that at least some of the relative movement takes place in a planar or curved locus conforming to the planar or curved interface between the two overlapping strap portions.
A variety of mechanisms have been proposed for effecting the relative motion necessary to friction-fusion welding techniques. Signode Corporation, 3600 West Lake Avenue, Glenview, Ill. 60025 U.S.A. (the assignee of the present invention), has developed and currently markets friction-fusion strapping machines which incorporate torsion bar assemblies. The torsion bar assembly is initially stressed and subsequently released to oscillate the strap engaging member, as well as the overlapping strap portion contacting the strap engaging member, for thus effecting the necessary relative motion.
U.S. Pat. Nos. 3,494,280 and 3,548,740, also assigned to Signode Corporation, disclose such torsion bar assemblies in strapping machines. The torsion bar assemblies described in these patents have been further developed and subsequently incorporated in strapping machines marketed by Signode Corporation. Typical of such commercially available machines are those sold under the designations "Power Strapping Machine Models MLN-2A, ML2-EE, ML2-JE, and ML2-HG."
Although the above-described power strapping machines with torsion bar assemblies work well in the many applications for which they were designed to be employed, the inventor of the present invention has determined that it would be desirable to provide an improved torsion bar assembly for use in power strapping machines. The inventor has determined that if a conventional torsion bar assembly design is employed with a relatively short stroke (small oscillation amplitude), then the assembly must be massive enough to accommodate the energy needed to properly form the joint. However, a very short stroke (small oscillation amplitude) is more easily damped out before a sufficient melting of the interface region between the overlapping strap portions can be effected. Although this tendency may be overcome by using a relatively large diameter torsion bar with a small oscillation amplitude, more massive structural supports are then required to accommodate such a design. The inventor of the present invention has determined that, consistent with minimal supporting structure, it is preferable to use a smaller diameter torsion bar with a larger twist angle than to use a larger diameter torsion bar with a smaller twist angle.
In conventional torsion bar assemblies, the strap engaging member that is oscillated by the torsion bar is part of, or is secured to, the torsion bar. The strap engaging member includes an arm projecting radially outwardly from the torsion bar. With a relatively small diameter torsion bar and with a relatively large twist angle, such a strap engaging member oscillates with a relatively large amplitude. However, in some applications, the large oscillation amplitude causes an unduly large movement of, and tension impact upon, the engaged strap.
The impact can be especially significant at the corners of the article around which the strap has been tensioned. This impact, of course, can place an undesirably high stress on the strap at the corner locations and may also damage the strap and/or article. Accordingly, it has been determined that, in some applications, a reduction in the amplitude or stroke of the strap engaging member would be desirable so as to substantially reduce, if not altogether eliminate, such problems.
As a way to reduce the stroke, the inventor of the present invention has considered locating the strap contacting surface of the strap engaging member as close as possible to the longitudinal axis of rotation of the torsion bar. For example, a design might be considered wherein the radially projecting arm of the strap engaging member is eliminated and wherein the strap engaging member comprises the exterior cylindrical surface of the torsion bar per se. With such a design, the actual amplitude of strap oscillation will be considerably reduced even though the twist angle of the torsion bar can remain relatively large (as desired from the above-discussed standpoint of providing sufficient energy for the friction-fusion weld in a manner that will produce a good weld and yet not require excessive supporting structure for the torsion bar).
Although the above-postulated design of a torsion bar assembly appears, in theory, to provide a solution to the problem of effecting a good weld with sufficient energy and reduced strap movement amplitude, the design suffers from practical drawbacks arising from conflicting design considerations. In particular, with commercial strapping apparatus, especially with strapping machines that automatically encircle an article with the strap, sufficient space must be provided at the friction-fusion welding location to accommodate the initial proper positioning of the overlapping strap portions and the subsequent tensioning of the strap. In addition, sufficient room must be provided to accommodate the various support structures and mechanisms for initially forming the strap into the loop, for gripping one or more portions of the strap, for stressing the torsion bar assembly, and for cutting the welded strap loop from the trailing portion of the strap.
Accordingly, the strap engaging member in a conventional torsion bar strapping machine typically extends radially outwardly from the torsion bar so that the strap engaging member contacts the strap at a location spaced away from the torsion bar by an amount sufficient to provide the required clearance for accommodating the above-described various functions. Unfortunately, with such a design, the amplitude of oscillation of the strap engaging member, and of the engaged strap, will necessarily be relatively large when the torsion bar is stressed through a relatively large twist angle. It would therefore be desirable to provide some mechanism for reducing the amplitude of the oscillation of the strap engaging member (and of the engaged strap) while permitting a relatively small diameter torsion bar to be stressed through a relatively large twist angle.
With the conventional torsion bar friction-fusion strapping machines described above, the strap engaging member has a convex strap gripping surface which presses the two overlapped strap portions against an anvil having a generally concentric and concave bearing surface. The strap engaging member and anvil are located in each machine at the bottom of the package or article receiving area so that the bottom of the package or article overlies, and is in close proximity to, the anvil and strap engaging member. The anvil is disposed between the bottom of the article and the strap path so that the overlapping strap portions can be pressed into the concave bearing surface of the anvil by the convex strap engaging member.
With many articles, the bottom of the article is flat and the portion of the tensioned strap extending around the bottom of the article would necessarily tend to conform to the flat bottom of the article. However, because the anvil and strap engaging member are curved, the strap must follow the curved, and longer, path defined between the anvil and the strap engaging member. Thus, after the friction-fusion weld has been completed and the strapped article removed from the machine, there will be a slightly greater amount of slack in the tensioned loop compared to a lesser amount of slack that would be introduced by a flat anvil.
Owing to the generally elastic nature of thermoplastic strap, the small amount of additional slack introduced by the curved anvil is manifested in a slightly reduced tension in the strap loop. This, of course, can be accommodated by drawing a higher initial tension. However, from the standpoint of minimizing the machine power requirements and the strap tensile strength requirements, the inventor has determined that it would be desirable to reduce, if not eliminate altogether, the concave curvature of the anvil and conforming convex curvature of the strap engaging member.
Further, the inventor of the present invention has determined that it is desirable, especially in welding overlapping portions of very thin strap (film strap), to reduce the amount of curvature at the weld region so as to be able to provide a weld extending for a greater distance along the length of the strap than would otherwise be achievable on a practical basis with a conventional curved weld region.
Accordingly, it would be advantageous to provide a torsion bar assembly with a strap engaging member that would move in a relatively linear path against a flat anvil parallel to the bottom of the article rather than in the conventional path curving away from the article.
In the above-described torsion bar strapping machines the strap is initially automatically formed into a loop about the article by a loop-forming system. The system typically includes a main chain assembly supported in a suitable framework to define a generally rectangularly-shaped window or region around the article receiving station in the machine. Fixed to the main chain assembly is a slip-feed strap carrier, such as a pair of spaced-apart rollers, which engages the trailing portion of the strap in a slip-feed manner and which is moved by the main chain assembly around the article to form the strap into a tight loop around the article. An example of such a system is disclosed in the aforementioned U.S. Pat. No. 3,548,740. A modified system is disclosed in the recently allowed U.S. Pat. Ser. No. 261,969 filed on May 8, 1981 and assigned to the assignee of the present invention.
The carrier main chain assemblies in such conventional systems are of the well-known multiple strand, metal link and pin type that are guided, as well as driven, by conventional metal sprockets. Although such conventional carrier chain systems work extremely well, abnormally high speed operation of the metal chain assemblies around metal sprockets can produce high noise levels.
In order to increase the efficiency of automated assembly line article strapping, it has become desirable to provide strapping machines with higher and higher operating speeds. However, when strapping machines with metal carrier chains and sprockets are operated at such very high speeds, the operational noise levels become excessive.
In the United States of America the noise levels in work areas occupied by employees are limited by various state and federal regulations. Accordingly, it would be desirable to provide a strap transport system in a power strapping machine that would have the strength and durability of a metal chain but that would have the relatively low noise level associated with nonmetallic belt drive assemblies. Further, with such an improved strap transport system, it would be desirable to provide a structure that could be relatively easily fabricated.
Although conventional strap works well in a great many applications, the inventor of the present invention believes that it would be highly desirable to provide, in certain special applications, strap that is considerably wider than conventional strap (e.g., two to eight times as wide) and that is considerably thinner (e.g., less than 0.254 mm and typically about 0.08 mm.).
Such strap could advantageously be used in certain applications, including the binding of a stack of newspapers or magazines, and, preferably, may also be transparent. The relatively wide strap would reduce the pressure on the stack of newspapers or magazines, particularly at the corners, and would thereby have less of a tendency to damage the newspapers or magazines. Finally, a relatively thin, transparent, strap readily permits viewing of the portion of the article which is covered by the strap.
With conventional thermoplastic strap having thicknesses of between about 0.254 mm. and about 0.889 mm., the overlapping strap portions are bonded together in a friction-fusion weld to a thickness of between about 0.013 mm. and 0.051 mm. in each overlapping strap portion across the entire width of the strap. Typically, the length of the friction-fusion bond extends for about 10 mm. to about 35 mm. along the length of the overlapping strap portions.
The inventor of the present invention has determined that conventional friction-fusion techniques developed for strap of conventional thickness are difficult to employ satisfactorily with thin film wide strap, especially film strap having a thickness of less than 0.13 mm. and which may be only about 0.08 mm. For one thing, much more energy would be required to melt the entire surface areas of the overlapping wide strap portions in the selected joint region. Further, control of the thickness of the fused material in the thin strap would be difficult. Also, care must be taken to avoid unwanted penetration of one or both of the overlapping strap portions.
The inventor of the present invention has found that additional, unique problems are presented by film strap fabricated from so-called "oriented" materials, such as, for example, strap comprising linear crystallization polypropylene that has been worked into a thin film having planar molecular orientation of the macromolecular chains with a uniplanar, axial oriented crystalline structure through at least a major portion of the film thickness. An attempt to produce a conventional friction-fusion weld in such film strap across the full width of the strap may result in reduced weld strength and can reduce the strap strength at the weld since the strap orientation is destroyed in the fused region of the weld.
The inventor has thus determined that it would be desirable to provide a method and apparatus for forming a friction-fusion joint or weld in overlapping portions of the thin film strap whereby the overlapping strap portions retain a sufficient amount of tensile strength after formation of the friction-fusion weld to enable the strap to properly function in a tensioned loop around an article at conventional strapping tensions for the applications in which such thin film strap would be used. Such an improved method and apparatus should desirably accommodate various means for effecting the bodily sliding frictional of the overlapping strap portions--including torsion bar actuated mechanisms as well as other non-torsion bar actuated mechanisms.